System and method for dynamically analyzing and improving the performance of a network

ABSTRACT

In a method and system for dynamically improving the performance of a server in a network, a tuning system monitors a workload of the server in real time, monitors a set of internal performance characteristics of the server in real time, and monitors a set of adjustable server parameters of the server in real time. The workload of the server may include the frequency and type of service requests received by the server from clients in the network. The internal server performance characteristics may include, for example, a data cache hit ratio of a data cache in the server. The set of server parameters may include, for example, the overall data cache size or the data cache geometry of the server. The tuning system periodically alters one or more of the set of adjustable server parameters as a function of the workload and internal performance characteristics of the server. Since the tuning system is continuously monitoring workload and performance characteristics of the server and altering the server parameters accordingly, the effectiveness of a given change in the server parameters is reflected in the next set of monitored workload and performance values.

This is a divisional of application Ser. No. 08/072,613 filed on Jun. 4,1993, now abandoned.

FIELD OF THE INVENTION

The present invention is directed to a computer network and provides adynamic method of analyzing and improving the performance of thenetwork. More specifically, the present invention is directed to asystem and method for improving the performance level of a networkserver by dynamically adjusting (i.e. tuning) the parameters of theserver in response to changes in the workload of the server.

BACKGROUND OF THE INVENTION

In a computer network, a client (e.g. a processor or computer in thenetwork) that wishes to obtain a particular service will generally havea wide range of available resources that are capable of performing theservice. While this is advantageous, it also creates resource managementproblems, e.g., which one of these resources should the client utilize?In order to more efficiently provide services to a client in a network,it is known in the art to provide a server. A server is a device,generally comprising both hardware and software, which acts as an agentbetween the client and one or more resources to deliver a requestedservice to the client.

A typical server is capable of providing a plurality of services to aplurality of clients. Changes in the type and quantity of servicesrequested necessarily affect the performance of the server by, forexample, increasing the response time of the server, i.e. the time ittakes the server to respond to a service request from a client.Moreover, the manner in which a server operates is, to a certain extent,dictated by a set of server parameters. The server parameters mayinclude, for example, the data cache buffer geometry, data cacheflushing rate, and open file cache timing window of the server. If theperformance of a server is unacceptable, the performance may be improvedby changing the server parameters in accordance with a tuning process.

In prior art systems, the server was tuned in a static manner.Specifically, the process of tuning required a person with extensiveexperience with the server to gather information from the system,determine what caused a performance degradation, and then proposechanges in the server parameters. The server must then be shut down andrestarted with the new set of parameters. Data must then be gathered andanalyzed again in order to determine if the changes had the desiredeffect. If the performance is still unsatisfactory, the process must berepeated.

This method has obvious disadvantages. It not only requires the personalattention of a highly trained individual but also involves a timeconsuming process of trial and error that requires shutting down andrestarting the server. It is an object of the present invention to solvethese problems by providing a dynamic system for automatically tuning aserver which does not require a server shutdown.

SUMMARY OF THE INVENTION

The present invention provides a system and method for dynamicallyimproving the performance of a server in a network. The presentinvention accomplishes this task by dynamically changing the parametersof the server in response to changes in the server's workload.

In general, the basic data used in a performance analysis of a serveris:

a) What is the work the server is asked to perform?

b) When are the clients receiving unacceptable service?

c) What are the values of the parameters of the server?

d) How do a), b), and c) change with respect to time?

In accordance with the present invention, a tuning system coupled to theserver monitors the values of the serve's parameters to form a state ofthe server and monitors the values of the server's workload to form astate of the workload. In addition, the tuning system monitors serverperformance. Server performance monitoring can be performed on twolevels. On the first level, the tuning system monitors the server'sinternal functions by retrieving information regarding internalperformance characteristics such as the data cache hit ratio. On thesecond level, the tuning system periodically makes service requests tothe server and measures the server's response time with respect to theserequests.

In accordance with the present invention, if the tuning systemdetermines that the performance of the server is unacceptable, it willautomatically alter the server parameters (the server state), takinginto account the server's workload, in order to improve the server'sperformance. If the tuning system determines that the server'sperformance cannot be improved by a change in the server state, it willset an alarm to alert the system manager to the problem.

In accordance with the present invention, the tuning system monitors theworkload of the server, the server parameters, and the server'sperformance. The tuning system utilizes the relationship between serverparameter, server workload, and server performance to automatically andcontinuously match the server's state to the workload it must service inorder to maintain an acceptable level of performance.

In accordance with an exemplary embodiment of the present invention, thetuning system allows for internally generated performance thresholds aswell as user defined performance thresholds. The tuning system monitorsthe server's performance and changes the server's parameters to insurethat the server remains within the permissible thresholds despiteworkload changes. This feature allows the user to define a higher (orlower) level of performance for the server depending upon the needs ofthe network.

In accordance with the present invention, the tuning system operates tocontinually match the server state to the workload demand which theserver must service. In order to accomplish this task, the tuning systemutilizes a set of micro-level algorithms and a set of macro-levelalgorithms.

The micro-level algorithms perform dynamic tuning by recognizing changesin the workload state (or monitored performance values) and, in responsethereto, applying appropriate changes to the server parameters in anattempt to mediate adverse performance effects. Micro-level algorithmsoperate on those server parameters which explicitly address singularbehaviors in the server, such as the data cache flushing rate, the openfile cache time window, the data cache geometry of buffers, etc.

In accordance with an embodiment of the present invention, a set ofmicro-level algorithms is provided for each tunable parameter of theserver. In general, a micro-level algorithm effecting changes in one ofthe server parameters is intended to affect only that parameter. Forexample, the micro-level algorithm which targets the open file cachetime window takes in only arguments which are inherent in the design andfunction of the open file cache. It therefore effects changes in thisparameter based solely on this data, not intending to explicitly effectchanges in other parts of the server (although this might occur undercertain circumstances). The dynamic tuning of the server is based upon afoundation of several such micro-level algorithms, all workingindependently on various sub-systems in the server and trying todynamically match state transitions in the workload toperformance-compensating transitions in each targeted sub-system.

In an embodiment of the present invention, the micro-level algorithmstarget the open file cache and data cache of the server. With regard tothe open file cache, the tuning system monitors server performancecharacteristics and alters server parameters relating to the hashingtable and close file delay. With regard to the data cache, the tuningsystem monitors server performance characteristics and alters serverparameters relating to the hashing table, data cache size, data cachegeometry, and data cache flushing rates.

The macro-level algorithms relate to the overall state of the server andworkload rather than the individual server parameters. The changes inserver parameters made by the micro-level algorithms translate intochanges in the server state. The macro-level algorithms map the changesin the server state to changes in the server workload and in theserver's performance in responding to that workload. In this manner, themacro-level effect of the micro-level dynamic tuning translates into anobservable dynamic tuning of overall server performance which maps astate transition of the server to a corresponding state transition inthe workload.

The macro-level algorithms create a representation of overall behaviorof the server from the micro-level data and use this behavior to addressmacro-level phenomena such as performance and capacity saturation. Thisrepresentation of overall server behavior is also used to allow a systemuser to monitor the behavior of the server as a whole and theeffectiveness of the micro-level algorithms in tuning the server.

In accordance with an exemplary geometrical modelling technique, thetuning system uses macro-level algorithms to create a three dimensionalsurface which represents the relationship between the server's workload,the amount of server resources consumed by the workload, and theperformance of the server in response to the workload. In accordancewith this embodiment, an enumeration technique provides unique valuesalong a first axis representing the server's workload. A system resourcecharacteristic (e.g. CPU utilization) is plotted along a second axis foreach unique workload value and a server performance metric (e.g. theserver's average response time to client requests) is plotted on a thirdaxis for each unique workload value. Thus, the performance and resourceutilization of the server, for all possible workload values, is plottedto create a three dimensional representation, or surface topology, ofthe server.

The tuning system monitors the workload, resource utilization, andserver performance and maps the current values onto the surface topologyof the server to determine the current position of the server on thesurface topology. As the workload changes, the current position of theserver on the surface topology also changes. Therefore, as the workloadchanges over time, the system according to the present invention tracksthe motion of the server over the surface topology and displays it tothe user. Moreover, since the server's surface topology represents theentire universe of possible resource/workload/performance combinations,it is possible, by examining the server's recent positions on thesurface topology, to identify not only the currentresource/workload/performance information but also to predict, to acertain extent, future performance information based upon trends in theworkload.

In another embodiment of the present invention, the macro-levelalgorithms are used to track the entropy of the server. Ideally, theresponse time of a server to a particular service request should notvary over time. To the extent that the server's response time does varyover time, the server is demonstrating entropic behavior. The tuningsystem measures the response time of the server by performing serverprimitive operations on the server and measuring the server's responsetime. As will be discussed further below, the tuning system may also usea set of probes located in clients in the network to measure theserver's response time in the same manner. The tuning system derives avalue for the entropy of the server from these measured response timevalues and plots these entropy values on a graph as a function of time.If the system detects an abrupt increase in the entropy in a short timeinterval (i.e., saturation behavior), the system will set an alarm tothe user.

In a further embodiment of the present invention, the tuning systemanalyzes the response times of the server and warns the user when theresponse times are beginning to exhibit saturation behavior. The tuningsystem determines when a server is approaching saturation behavior byplotting "triplets" of measured response times (RT₁, RT₂, RT₃), i.e.three consecutive response times, on a three dimensional graph. Aresponse time saturation value RTsat is defined as the response time atwhich the server begins to exhibit saturation behavior. The maximumpermissible response time is plotted on the graph as a cube having sidesof length RT_(sat). It has been empirically determined that the set oftriplets measured over time will typically be bounded by a sphere ofradius r_(b). In accordance with the present invention, the rate atwhich the sphere is approaching the cube of length RT_(sat) isidentified and reported as the rate at which the server is approachingsaturation behavior (i.e. one hour to saturation). As a result, thetuning system can set an alarm indicating that the server is approachingsaturation behavior when this rate drops below a threshold value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a network including a tuning system according to thepresent invention.

FIG. 2 illustrates a hashing table.

FIG. 3 is an illustrative computer program for implementing Open FileCache Hashing Table Micro-algorithm according to the present invention.

FIGS. 4(a-f) is an illustrative computer program for implementing a DataCache Size Micro-algorithm according to the present invention.

FIG. 5 illustrates a histogram according to the present invention.

FIGS. 6(a-c) is an illustrative computer program for implementing a DataCache Homogeneous Geometry Micro-algorithm according to the presentinvention.

FIGS. 7(a-c) is an illustrative computer program for implementing a DataCache Heterogeneous Geometry Micro-algorithm according to the presentinvention.

FIGS. 8(a-b) is an illustrative computer program for implementing a DataCache Flushing Micro-algorithm according to the present invention.

FIG. 9 is an illustrative computer program for implementing a Data CacheBuffer Starvation Micro-algorithm according to the present invention.

FIG. 10 is a graph illustrating the manner in which the system accordingto the present invention monitors the entropy of a server.

FIG. 11 is a graph which illustrates the manner in which a server'sperformance varies with its workload.

FIG. 12 is a graph which illustrates a server saturation alarm system ofthe present invention.

FIG. 13 is a table which illustrates response time triplets used in thegraph of FIG. 12.

DETAILED DESCRIPTION OF THE DRAWINGS

A tuning system 1 according to the present invention is illustrated inFIG. 1. The tuning system 1 uses a tuning engine 40 to dynamically tunethe parameters of a server 30 in response to changes in the server'sworkload. The server 30 typically resides in a host system 60 and iscoupled to a plurality of clients 10 in a client population 20 and toone or more resources 50. The Tuning engine 40, which is coupled to theserver 30, monitors the workload and internal performancecharacteristics of the server 30, periodically determines the responsetime of the server to service requests, and changes the serverparameters of the server in response to changes in the workload andinternal performance characteristics.

The data gathered during the tuning process, which is stored in a memory80 of the host system 60, is also used to generate a graphicalrepresentation of server performance as a function of server capacityand workload, and to alert a user to performance and capacitysaturation. In order to accomplish these tasks, the tuning engine 40utilizes a set of micro-level algorithms and a set of macro-levelalgorithms.

I. Micro-level Algorithms

In accordance with an embodiment of the present invention, the tuningengine 40 tunes the server through the use of several sets ofmicro-level algorithms. The micro-level algorithms perform dynamictuning by recognizing changes in the workload and internal performancecharacteristics of the server and, in response thereto, applyingappropriate changes to the server parameters in an attempt to mediateadverse performance effects. Micro-level algorithms operate on thoseparameters which explicitly address singular behaviors in the server.Therefore, an algorithm effecting changes in one of the parameters doesnot intend to explicitly effect changes in other parts of the server(although this might occur in certain situations).

There are many server parameters which may affect the performance of aserver. In accordance with an embodiment of the present invention,several server parameters, including, e.g., data cache flushing rates,open file cache timing windows, and data cache geometry, togethercompose the state of the server. This does not imply that these are theonly parameters which can define the state of a server system. Indeed,different types of server's may have different sets of serverparameters. Moreover, with regard to any particular server, some or allof the server parameters which affect server performance may be used inthe tuning process.

In accordance with an illustrative embodiment of the present invention,however, the data cache flushing rates, open file cache timing windows,data and open file cache hashing tables, and data cache geometrycomprise the representation of the server's state for which a mapping tothe workload has been identified.

Each of the server parameters which comprise the server state has beenfound to be amenable to dynamic tuning based on state transitions of theworkload. Each of these tunable server parameters has a correspondingalgorithm (or set of algorithms) which governs its value and transitionsequence. When collected as a set, these parameters define the serverstate for purposes of dynamic tuning. Each algorithm, while unique tothe particular type of target system, shares the following basicstructure:

    ______________________________________                                        : : : > an observable perturbation in the workload                            : :                                                                           : :  : : > causes : : >                                                       : :    an identifiable change in the parameter which                          : :    tends to compensate for the perturbation                               : :                                                                           : :    : : : > feedback on actual results of the                              : :       compensating action > : : : : :                                     : :                   : :                                                     : : : : : : : : : : : : : : : : : : : : : : : : : : : : : :                   ______________________________________                                    

Each of the micro-level algorithms contains a "sampling interval" ortime during which needed statistical data are retrieved from the serverby the tuning engine 40 in a non-reduced state and prepared for use bythe micro-level algorithms. Thus, a sampling interval of two secondsindicates that data are sampled from the server every two seconds. Eachmicro-level algorithm also contains an "action interval" or time inbetween transmission of tuning recommendations from the tuning system 1to the server, i.e. the processing time of the algorithm. An actioninterval of five seconds, for example, indicates that the parameter isupdated every five seconds. The sampling interval and action intervalfor a given micro-algorithm is a matter of design choice to be selectedby the system designer. The specific intervals indicated in thediscussion that follows are merely exemplary values which have beenfound to be acceptable.

A. RTMP's: Real-Time Monitoring of Critical Values

The tuning engine's micro-level algorithms depend upon having real-timemonitoring data concerning the server's internal behavior. This isaccomplished through the actions of the Real Time Monitoring Processes(RTMP's). The server systems which are targeted by the tuning engine 40are configured in such a way that the salient statistics needed by themicro-level algorithms's are constantly sampled and made available tothe tuning engine 40.

The various server sub-systems monitor the needed statistics andperiodically communicate the data to the Server Monitor Process (CMT₋₋SMP 70) which reduces data when necessary and stores the data in thememory 80 of the host system 60. The Central Client Process (CMT₋₋ CCP90), which resides in a Microsoft Windows program on the tuning engine40, communicates with the CMT₋₋ SMP 70 via a communication protocol(e.g. NetBios). In this manner, CMT₋₋ CCP 90 periodically assimilatesthe data from the RTMP's into the micro-algorithms. Once the algorithmsact on the data, the changes deemed necessary are communicated back tothe CMT₋₋ SMP 70 which places them into memory 80. The changes are thenperiodically communicated back to the server.

In the server 30, the changes recommended by algorithms are implemented,thereby effecting state transitions in the server. In the discussionthat follows, it should be understood that any values retrieved from theserver (e.g. data cache hit ratio, number of sequential reads) areretrieved and processed by RTMP.

B. Open File Cache Micro-level Algorithms

The open file cache is a mechanism in the server which attempts toimprove performance on file open requests by maintaining pertinent fileheader information in memory after a particular client has requestedthat the file in question be closed. This is accomplished by actuallykeeping the file open in the file system while telling the client whichrequested the file close operation that the close was actuallyperformed. In effect, the actual file close operation is delayed acertain period of time (designated the "close delay time interval") bythe open file cache. When a client closes a file and then reopens thefile during the close delay time interval, the server avoids the expenseof an actual file open because it has retained the pertinent informationin memory.

The data structures which contain the file header information aremaintained in memory buffers which are accessed through a hashing table.Referring to FIG. 2, a hashing table mechanism 100 allows for chainingof a plurality of buffers 110. Each buffer 110 contains the file headerinformation for one file. When the server wishes to access the fileheader information for a particular file, the file header I.D. is usedto access a pointer which indicates which slot 120 of buffers containsthe file header information. Once the slot 120 has been identified, thechain of buffers 110 for that slot must be searched in order to find thebuffer 110 which contains the file header information.

The open file cache is tuned by two micro-level algorithms OFC₋₋ 1 andOFC₋₋ 2. These algorithms can be implemented with a sampling interval of60 seconds and an action interval equal to five sampling intervals (300seconds).

In analyzing the performance of the hashing table, there are severalstatistics of interest. The load factor, (a), is defined as the averagenumber of elements stored in a chain (a=n/m), where

n=number of file header structure elements stored by the hashing table;

m=number of hash table slots 120 used to store the file header structureelements.

The average number of elements that must be examined in a successfulsearch, (n_(s)), is defined by the equation:

    n.sub.s =1+(a/2)-(1/2m ).

OFC₋₋ 1 attempts to tune the server by controlling the growth of n_(s).

With regard to the close delay time window, i.e. the amount of time afile is kept open after a client requests that it be closed, thefollowing variables are of interest:

File Close Delay (W_(d)):

the time interval (in seconds), measured by the server from the time theclient requests that a file be closed, during which the file is actuallykept open with the file header information resident in memory (the openfile cache). After the expiration of this interval, if no intercedingrequest to re-open the file has been processed, the file is actuallyclosed by the server. If a request does come in to reopen the file, thefile close delay is reset.

Number of Open Files (F_(o)):

the moving average (over the five most recent sampling intervals) of thetotal number of files which the server has maintained as open on behalfof client requests. For any particular sampling interval in the windowof this moving average: F_(o) =F_(i) +F_(a)

Number of Idle Files (F_(i)):

the moving average (over the five most recent sampling intervals) of thenumber of files which the server has maintained as open during the lastsampling period, but which have actually been requested to be closed bythe clients. These files are the contents of the open file cache.

Number of Active Files (F_(a)):

the moving average (over the five most recent sampling intervals) of thenumber of files which the setter maintained as open during the lastsampling period, and which have not been closed by clients during thatperiod.

Number of idle files which changed from idle to active (F_(IA)):

the moving average (over the five most recent sampling intervals) of thenumber of files which have moved from the idle list to the active list.The rate at which this motion occurs is the tuning system's measure ofthe effectiveness of the open file caching scheme in the server.

1. Open File Cache Hashing Table AlGorithm (OFC 1)

OFC₋₋ ALG1 monitors and acts upon the open file cache hashing table'seffectiveness by monitoring the variable, n_(s) (the average number ofelements that must be examined in a successful search). The tuningsystem 1 samples n_(s) periodically and computes a value n_(s-avg-t),which is the moving average of n_(s) over the five most recent samplingintervals. It also maintains an internal representation of the map ofn_(s-avg-t) as a function of time. When the tuning engine 40 detectssaturation behavior in the values of n_(s-avg-t), OFC₋₋ ALG1 attempts tomove n_(s-avg-t) back into a linear region. Under normal conditions,n_(s-avg-t) will remains relatively constant over time. As thesaturation value is approached, however, n_(s-avg-t) will suddenly grow.This saturation value is sometimes referred to as the "knee of thecurve" and the manner in which it is detected is discussed below inSection II(c) and in copending application Ser. No. 08/050,929 which isincorporated herein by reference. In any case, when OFC₋₋ ALG1 detectssaturation behavior, it attempts to compensate by changing the number ofhash table slots (m).

Referring to the illustrative computer program of FIG. 3, OFC₋₋ ALG1periodically samples n_(s) and generates a value n_(s-avg-t) (OFC₋₋STATS), which is the moving average of n_(s) during the five most recentsampling intervals measured from time "t". This value is periodicallyplotted on a graph as a function of "t" (maintain₋₋ ofc₋₋ hash₋₋graph(OFC₋₋ STATS)). Each time a new value of n_(s-avg-t) is plotted,OFC₋₋ ALG1 checks the graph to see if n_(s-avg-t) is exhibitingsaturation behavior (check for danger()). If saturation behavior isdetected, OFC₋₋ ALG1 selects a value of n_(s-avg-t) from a recent"linear" portion of the n_(s-avg-t) vs. time curve and, using:

a) this "linear" n_(s-avg-t) ; and

b) the most recently sampled value for n; solves the n_(s) equation form (new₋₋ value₋₋ m). This new value for m is sent to the server viaCMP₋₋ SMP and the server implements the change during its next statetransition (send₋₋ new₋₋ m ₋₋ to server()).

2. Open File Cache Close Delay Algorithm (OFC ALG2)

OFC₋₋ ALG2 monitors and acts upon the open file cache's file close delayvalue (W_(d)) which is the amount of time the cache keeps a file openafter receipt of a file close request from a client. The servermaintains an internal default value for W_(d) (W_(do)). In accordancewith OFC₋₋ ALG2, the tuning engine 40 monitors, during each actioninterval, the total number of open files (F_(o)), the number of openfiles that are idle (F_(i)), the number of open files which are active(F_(a)), and the number of idle files that have become active (F_(IA)),and manipulates W_(d) accordingly. In accordance with the algorithm, theopen file cache is placed in one of 5 defined "cases"(a-d) as follows:

    ______________________________________                                                 F.sub.o                                                                           F.sub.i      F.sub.a                                                                             F.sub.IA                                      ______________________________________                                        a)         =0    =0           =0  =0                                          b)         >0    =0           >0  =0                                          c)         >0    >0           >0  =0                                          d)         >0    >0           >0  >0                                          e)         >0    =0           >0  >0                                          ______________________________________                                    

If the open file cache is in case (a), there has been no file openactivity during the action interval (t). This is the trivial case inwhich OFC₋₋ ALG2 sets the value for open file close delay during thenext action interval to be the default value: W_(d) (t+1)=W_(do)

If the open file cache is in case (b), then all the open files duringthis action interval were in the active state. None of the files wereidle. Therefore, open file caching was not a factor during this actioninterval because none of the clients requested that an open file beclosed. In this case, OFC₋₋ ALG2 will set the file close delay duringthe next interval to be the default value: W_(d)(t+1) =W_(do)

If the open file cache is in case (c), then some of the open files havebeen closed by clients during the previous action interval. The openfile cache, however, has kept these files "open" during the close delaytime period in the hope that these files might be re-opened before theclose delay period expired. Since there has been no idle->active motionduring the previous action interval, however, none of the closed fileswere reopened during the close delay period.

This situation could indicate the close delay time period is too short.To test this hypothesis, OFC₋₋ ALG2 checks to see if this instance ofcase "c" is one of a sequence of three (3) successive instances of case"c" for which it is testing this hypothesis. If this is the situation,then OFC₋₋ ALG02 will increase the value of the close delay interval bya random integral multiple of the default value such that W_(d) (t+1)=random; wherein W_(d)(t) <random<=(5* W_(do)).

If, however, this instance of case "c" is one of a sequence of at leastfive previous successive instances of case "c" in which W_(d) was set toa random integral multiple of W_(do) (as defined above), then OFC₋₋ ALG2will conclude that the workload is not amenable to open file caching andwill reset the close delay to the default setting: W_(d) (t+1) =W_(do).

If the open file cache is in case (d), then some of the open files wereactually closed by the clients; kept open in an idle state; and thenreopened by the clients during the close delay period. In thissituation, a longer close delay might produce a still higher rate ofidle->active motion. To explore this possibility, OFC₋₋ ALG2 willincrease the close delay value by a random amount (W_(d) (t+1) =random *W_(d0), where W_(d)(t) <random <=(5* W_(d0))), unless it had previouslyattempted an increase during the last interval. If an increase wasattempted in the last interval, the close delay is left unchanged (ifW_(d)(t-1)< W_(d) (t), then W_(d)(t+1) =W_(d)(t)).

If the open file cache is in case (e), then every file that was closedby a client during the previous action interval, and kept open by theopen file cache, was reopened by the client before the expidelay. Inthis cose delay. In this case, OFC₋₋ ALG2 determines that the closedelay is set at its optimal value (since there are only active fileswith motion from the idle to active list and no files left over on theidle list) and W_(d) (t+1)=W_(d) (t).

C. Data Cache Micro-algorithms

The data cache is one of the most important performance enhancementmechanisms in the server. It is tunable on several parameters: its size,its buffer geometry, its hashing table, and its flushing intervals. Thetuning system 1 periodically samples specified statistics of interest(detailed below) and not only maintains values for these statistics foreach sampling interval but also maintains a moving averages of thesestatistics over a window of the five most recent sampling intervals.

The data cache is a system of discrete regions of mapped memory (datacache buffers) into which data can be read or written. Therefore,increasing or decreasing the data cache size is effected by increasingor decreasing the number of data cache buffers which comprise the datacache.

The cache geometry of a data cache can be either homogeneous ofheterogeneous. A data cache with a homogenous geometry is comprised of aset of cache buffers of identical sizes (in bytes). In contrast, a datacache with a heterogenous geometry is comprised of a set of cachebuffers of non-identical sizes such that the overall cache is broken upinto several regions, each region characterized by a different buffersize. Within each region of a heterogeneous data cache, the geometry ishomogenous, but across regions the geometry is hererogenous.

1. Data Cache Hashing Table Algorithm (DC ALG1)

The data cache, like the open file cache, has a hashing table which isamenable to dynamic tuning. DC₋₋ ALG1 monitors and acts upon the datacache hashing table's effectiveness in precisely the same manner inwhich OFC ALG1 acts upon the open file cache hashing table.

2. Data Cache Cache Size Algorithm (DC ALG2)

a. Homogenous Data Cache

DC₋₋ ALG2 will be initially described with reference to a homogenousdata cache. DC₋₋ ALG2 acts on the data cache statistics relating to thedata cache size, and on input from DC₋₋ ALG5 regarding cacheavailability, to determine the optimal number of buffers which shouldmake up the data cache. In DC₋₋ ALG2, only the overall cache size isvaried. The cache geometry is kept constant. DC₋₋ ALG2 has a samplinginterval of 60 seconds and an action interval equal to 60 samplingintervals (1 hour). The following variables can be used to implementthis algorithm:

Maximum size for data cache S_(max) :

the maximum size to which the data cache can be grown. This value iseither calculated by the tuning system 1 during the initial systemconfiguration or is a user-defined value.

Current size of the data cache S_(c) :

the current size of nhe data cache in bytes (the sum of the sizes of allthe data cache buffers).

Maximum number of buffers in the data cache (homogenous cache) N_(ht) :

the total number of buffers in a homogenous data cache (all buffersidentical size).

Size of a buffer in an homogenous data cache B_(h) :

the size in bytes of a buffer in a homogenous data cache (all buffers incache identical size).

Maximum number of buffers in the small buffer pool (hererogenouscache)N_(s) :

the total number of buffers in the small region of a beterogenous datacache (all buffers in the small region are identical size).

Maximum number of buffers in the medium buffer pool (heterogenous cache)N_(m) :

the total number of buffers in the medium region of a heterogenous datacache (all buffers in the medium region are identical size).

Maximum number of buffers in the large buffers pool (hererogenous cache)N₁ :

the total number of buffers in the large region of a hererogenous datacache (all buffers in the large region are identical size).

Size of a buffer in the small region of a heterogenous data cache B_(s):

the size in bytes of a buffer in the small region of a heterogenous datacache (all buffers in region of identical size).

Size of a buffer in the medium region of a heterogenous data cache B_(m):

the size in bytes of a buffer in the medium region of a beterogenousdata cache (all buffers in region of identical size).

Size of a buffer in the large region of a hererogenous data cache B₁ :

the size in bytes of a buffer in the large region of a heterogenous datacache (all buffers in region of identical size).

Data Cache Hit Ratio R_(avg) :

the moving average (over the five most recent sampling intervals) of thecache hit ratio, where the cache hit ratio is the ratio of number ofrequests for data to the number of those requests which were satisfiedby data in the data cache buffers.

Hit Ratio Window-1 R1 3!:

An array of the cache hit ratios from the last three sampling intervals.

Hit Ratio Window-2 R2 5!:

An array of the cache hit ratios from the last five sampling intervals.

Data Cache Hit Ratio R_(avg) small!:

the moving average (over the five most recent sampling intervals) of thecache hit ratio in the "small" buffer region of the hererogenous datacache.

Data Cache Hit Ratio R_(avg) medium!:

the moving average (over the five most recent sampling intervals) of thecache hit ratio in the "medium" buffer region of the hererogenous datacache.

Data Cache Hit Ratio R_(avg) large!:

the moving average (over the five most recent sampling intervals) of thecache hit ratio in the "large" buffer region of the henerogenous datacache.

As previously mentioned, DC₋₋ ALG2 acts on the data cache statisticsrelating to data cache size (which is sampled from the server by thetuning system 1) and from input regarding cache availability (which, asexplained infra, is obtained from DC₋₋ ALG5) in order to determine theoptimal number of buffers which should make up the data cache size.Referring to FIG. 4, DC₋₋ A LG2 uses the value R_(avg) (data cache hitratio) to place the data cache in one of the following four cacheperformance states (a-d):

a) R_(avg) <=50%

b) 50% <R_(avg) <=80%

c) 80% <R_(avg) <=90%

d) 90% <R_(avg) <=100%

Case (a) is treated as the degenerate state. The cache hit ratio isunacceptable and the cache size must be significantly increased (ifpossible). Therefore, when DC ALG2 detects a first instance of case (a)(case₋₋ a₋₋ cache-size()), it increases the number of cache buffers by100% (or to S_(max) if a 100% increase will exceed S_(max)). Thisprocedure is repeated for each successive instance of case (a) until:

1) the hit ratio improves enough to move the cache into another cacheperformance state (i.e., hit ratio improves >50%); or

2) the memory resources on the server system cannot support any furthercache size increases. In this case, an alarm (alarm (degenerate₋₋resources)) is set to alert the central user that the data cache cannotsupport the workload with adequate performance unless the server isequipped with adequate resources to increase the size of the data cache.

Case (b) indicates that the data cache has a "low" to "moderate" cachehit ratio (case₋₋ b₋₋ cache-size()). Therefore, DC₋₋ ALG2 issues aseries of gradual, rather than dramatic, cache size increases. It hasbeen empirically demonstrated that cache buffer management is ameasurable overhead for the server system which can adversely affectdata cache performance. Moreover, an increase in data cache size doesnot necessarily result in an increase in the hit ratio because theworkload-driven pattern of data cache accesses combined with the cache'sdata replacement mechanism might prevent the hit ratio from trackingcache size.

Therefore, increasing the size of the cache without regard to thecorresponding rate at which the hit ratio is increasing might result inboth an unnecessary use of resources and a decrease in cacheperformance. To avoid such a result, DC₋₋ ALG2 attempts modest (25%)successive increases in the number of cache buffers while simultaneouslymonitoring the resulting hit ratio changes (1.25* N_(ht)). If DC₋₋ ALG2determines that the modest increases in cache size during the last fivesampling intervals R2 5! have not produced any measurable increases inthe hit ratio, then it stops increasing the cache size until the hitratio begins to deteriorate, at which time incremental increases areresumed.

Case (c) indicates that the data cache has a "good" cache hit ratio(case₋₋ c₋₋ cache-size()). The algorithm used for case (c) is similar tothe implementation in case "b" with the following two differences:

1) The incremental increases are more "gradual", 10% rather than 25%;and

2) The cessation of incremental increases is more rigid. In accordancewith case (c), cache size increases are inhibited if the hit ratios hasfailed to increase in any of the last three successive samplingintervals R1 3!. Cache size increases resume, however, if the hit ratiohas failed to increase in all of the last three successive intervals.

Case (d) indicates that the data cache has an "excellent" cache hitratio (case₋₋ d₋₋ cache-size()). In case (d) DC ALG2 simply leaves thecache size at its current value.

As previously mentioned, DC₋₋ ALG2 also utilizes input relating to datacache availability in determining the optimal data cache size. Datacache availability is defined as the ratio of the number of times thedata cache had to be flushed in order to free up a data cache buffer tosatisfy a read or write request from a client to the total number ofreads and writes from clients during a sampling interval. DC₋₋ ALG5,described below and in FIG. 9, monitors the data cache availability andasynchronously sends DC₋₋ ALG2 a command instructing it to bypass itsnormal procedures and implement a specified case based upon the datacache availability monitoring conducted by DC₋₋ ALG5(CheckForStarvation). Thus, DC₋₋ ALG5 overrides the normal actioninterval of DC₋₋ ALG2 and forces DC₋₋ ALG2 to alter the cache sizebefore it normally would.

b. Heterogeneous Data Cache

With regard to a heterogeneous data cache, DC₋₋ ALG2 is appliedsimultaneously to each of the regions of the heterogeneous data cache.In this manner, each region is treated as a homogenous data cache withina larger heterogeneous super-set. As a result, the size of each regionis optimized in accordance with DC₋₋ ALG2 independent of the otherregions in the heterogeneous data cache.

3. The Data Cache Buffer Geometry Algorithms DC ALG3a & DC ALG3b

The data cache buffer geometry algorithms are designed to adjust theuniform cache buffer size in the homogenous cache (DC₋₋ ALG3a) and theregional cache buffer sizes in the hererogenous cache (DC₋₋ ALG3b) tooptimal sizes. The tuning system 1 performs real-time monitoring ofworkload (via RTMP) for use by these algorithms. These algorithms alsoutilize the real time data sampled by the tuning system 1 for use byDC₋₋ ALG2.

The tuning system 1 assimilates workload data measured during thesampling intervals into frequency distribution histograms for each ofthe following operations: sequential reads, sequential writes, randomreads, and random writes. These histograms map the frequency ofoccurrence of each operation into pre-defined byte-buckets. Thebyte-bucket sizes (or combinations of these sizes) provide the discretevalues which are considered by the algorithms as possible cache buffersizes. The result of these algorithms is to optimally map the buffercache geometry during a certain period of time to the workload stateduring that period of time. DC₋₋ ALG3a and 3b use a sampling interval of60 seconds and an action interval equal to 60 sampling intervals (1hour). In implementing their functions, these algorithms utilize thefollowing variables:

S_(b) : the size (in bytes) of a cache buffer in the homogenous cachegeometry.

S_(b) !: an array of the sizes (in bytes) of the cache buffers in thehererogenous cache geometry.

S_(b-sr) : the optimal cache buffer size for the sequential reads.

S_(b-rr) : the optimal cache buffer size for the random reads.

S_(b-sw) : the optimal cache buffer size for the sequential writes.

S_(b-rw) : the optimal cache buffer size for the random writes.

S_(b-srsw) : the optimal cache buffer size resulting from the merging ofS_(b-sr) and S_(b-sw) in the heterogeneous cache geometry.

SR_(max) : the size of the maximum byte bucket used in the sequentialread frequency distribution histogram measured for sequential readsduring the sample interval.

SW_(max) : the size of the maximum byte bucket used in the sequentialwrite frequency distribution histogram measured for sequential writesduring a sample interval.

RR_(mode) : the mode of byte-buckets in the frequency histogram(unimodal distribution) for random reads during a sample interval.

RW_(mode) : the mode of byte-buckets (unimodal distribution) for randomwrites during a sample interval.

RR_(mode) !: array of the modes of the byte-buckets (multi-modaldistribution) for random reads during a sample interval.

RW_(mode) !: array of the modes of byte-buckets (multi-modaldistribution) for random writes during a sample interval.

NM_(rr) : the number of modes (local peaks in the histograms) in therandom read histograms during a sample interval.

NW_(rw) : the number of modes (local peaks in the histograms) in therandom write histograms during a sample interval.

HIST₋₋ SR !: the frequency histogram of sequential reads during thesample interval provided by the tuning engine 40. In this histogram,frequencies of occurrence are maintained for random reads inbyte-buckets with values of 1)8 bytes (0-7), 2)16 bytes (8-15), 3)32bytes (16-31), 4) 64 bytes (32-63), 5)128 bytes (64-127), 6) 256 bytes(128-255), 7) 512 bytes (256-511), 8)1024 bytes (512-1023), 9)2048 bytes(1024-2047), 10)4096 bytes (2048-4095), 11)8192 bytes (4096-8191),12)16384 bytes (8192-+++).

HIST₋₋ RR !: the frequency histogram of random reads during the sampleinterval provided by the tuning engine 40. In this histogram,frequencies of occurrence are maintained for random reads inbyte-buckets of the same size as HIST₋₋ SR !.

HIST₋₋ SW !: the frequency histogram of sequential writes during thesample interval provided by the tuning engine 40. In this histogram,frequencies of occurrence are maintained for random reads inbyte-buckets of the same size as HIST₋₋ SR !.

HIST₋₋ RW !: the frequency histogram of random writes during the sampleinterval provided by the tuning engine 40. In this histogram frequenciesof occurrence are maintained for random reads in byte-buckets of thesame size as HIST₋₋ SR !.

PercentSR: the percent of the total reads and writes during a sampleinterval which were sequential reads.

PercentRR: the percent of the total reads and writes during a sampleinterval which were random reads.

PercentSW: the percent of the total reads and writes during a sampleinterval which were sequential writes.

PercentRW: the percent of the total reads and writes during a sampleinterval which were random writes.

a. Homogenous Geometry: DC₋₋ ALG3a

This algorithm determines the optimal size, S_(b), for a data cachebuffer when all data cache buffers must be the same size. The optimalcache size is a function of the type of data accesses requested of thecache (e.g. sequential reads v. sequential writes) and of the size ofthe data accesses (in bytes) within each of these types. For example,during a given sample interval, sequential reads tend to push theoptimal cache size (S_(b)) towards the value of the maximum byte bucket(SR_(max)) used in the sequential read frequency distribution histogramfor that sample interval. Similarly, sequential writes tend to pushS_(b) toward SW_(max). At the same time, the random reads are moving thevalue for S_(b) towards a value of RR_(mode) or a combination of thevalues of RR_(mode) ! (depending upon the frequencies of distribution).Finally, random writes are pushing the values of S_(b) toward RW_(mode)or a combination of RW_(mode) !. DC₋₋ ALG3a uses all of these values insynthesizing the optimum value for the data cache buffer size.

The tuning system 1 provides a histogram, updated each sample interval,for each of four operations: sequential reads, random reads, sequentialwrites, and random writes. Each histogram contains the frequency ofoccurrence of its respective operation categorized by byte-rangebuckets. Referring to FIG. 5, an illustrative histogram for sequentialwrites is shown with byte buckets s₁ -s₆. Each of the byte bucketscontains the frequency of occurrence, during the last sample interval,of sequential writes for a different range of byte sizes. For example,bucket s₁ might indicate the number of sequential writes of 0-7 bytesduring the last interval while bucket s₂ indicates the number ofsequential writes of 8-15 bytes during that same interval.

FIG. 6 shows an illustrative computer program for implementing DC₋₋ALG3a. Referring to FIG. 6, DC ALG3a determines the maximum byte bucketsize used in the sequential read frequency histogram for the sampleinterval (find₋₋ seq₋₋ read₋₋ max()) and sets an optimal value forsequential reads to that value (S_(b-sr) =SR_(max)). It then determinesthe maximum byte bucket size used in the sequential write frequencyhistogram (find₋₋ seq₋₋ write max()) for the sample interval and sets anoptimal value for sequential writes to that value (S_(b-sw) =SW_(max)).

The optimum value for random reads is based upon the mode(s) of thebytebucket ranges in the random read frequency histogram (find₋₋ num₋₋modes₋₋ rr(HiST₋₋ RR !). If the distribution is unimodal (NM_(rr) ==1),DC₋₋ ALG3a assigns the optimal value for random reads to the value ofthe mode's byte-bucket (S_(b-rr) =RR_(mode) =find₋₋ unimode(HIST RR !)).If the distribution is multimodal (NM_(rr) >1; i.e. local maxima), thealgorithm sets the optimal value to the byte bucket value correspondingto the mode with the highest frequency of occurrence in the histogramS_(b-rr) =find₋₋ best₋₋ multimode₋₋ rr(RR_(mode) !)). If there is a"tie," the larger byte bucket is used. If there are no modes in thedistribution, the byte bucket value associated with the median of therandom read distribution is used (optimal₋₋ size=median₋₋ rr=find₋₋median₋₋ rr(HIST₋₋ RR !)). The same procedure is used to find theoptimal value for random writes (see/* random writes */).

Thus, DC₋₋ ALG3a has provided four (4) optimal cache buffer sizes, oneoptimal value for each of the four types of operations (sequentialreads, random reads, sequential writes, and random writes). Since thisalgorithm operates on a homogenous cache geometry, these four valuesmust be reduced to a single optimal cache buffer size.

In order to derive this single optimal value, the algorithm must firstdetermine what percentage of the workload is associated with sequentialreads (PercentSR=determine₋₋ percent₋₋ sr()), random reads(PercentRR=determine₋₋ percent₋₋ rr()), sequential writes(PercentSW=determine₋₋ percent₋₋ sw();), and random writes(PercentRW=determine₋₋ percent₋₋ rw()). If there exists a mode in thefrequency distribution of the four operations (if (mode==unimodal), thenthe size of the cache buffer is set to the optimal cache buffer size forthe operation corresponding to the mode (S_(b) =optimal₋₋ size =size₋₋of₋₋ final₋₋ mode). For example, if the unique mode is defined bysequential reads, then the optimal cache buffer size is set to S_(b-sr).

If no single mode exists because all four operations show the samefrequency of occurrence, then the optimal cache buffer size is set tothe largest value from the four "optimal" values (optimal₋₋ size =find₋₋max₋₋ size(S_(b-sr),S_(b-sw), S_(b-rr),S_(b-rw))). For example, ifS_(b-sr) =1K, S_(b-rr) =512, S_(b-sw) =4K, and S_(b-rw) -1K, thealgorithm will set S_(b) =4K (the maximum value of the set of four).

If no single mode exists because three of the operations show the samefrequency of occurrence, then the optimal cache buffer size is set tothe largest value from these three "optimal" values (find maxsize(S₁,S_(b2),S_(b3))). For example, if S_(b-sr) =1K, S_(b-) =512, andS_(b-sw) =8K, the algorithm will set S_(b) =8K (the maximum value of theset of three).

If no single mode exists because two the operations show the samefrequency of occurrence, then the optimal cache buffer size is set tothe largest value from these two "optimal" values (find₋₋ max₋₋size(S_(b1),S_(b2))). For example, if S_(b-sr) =1K, S_(b-rr) =512, thealgorithm will set S_(b) =1K (the maximum value of the set of two).

b. Heterogeneous Geometry: DC₋₋ ALG3b

This algorithm determines the optimal size for a data cache buffer whencache buffers can be of differernt sizes. As in the homogeneous casedescribed above, sequential reads tend to push the optimal cache size(S_(b)) towards (SR_(max)), sequential writes tend to push S_(b) towardSW_(max), random reads push S_(b) towards a value of RR_(mode) or acombination of the values of RR_(mode) !, and random writes push thevalues of S_(b) toward RW_(mode) or a combination of RW_(mode) !.Moreover, DC₋₋ ALG3b, like DC₋₋ ALG3a, uses these values in its tuningoperation. Since the heterogeneous cache geometry allows the data cacheto be comprised of cache buffers of differing sizes, however, DC₋₋ ALG3buses these values to synthesize 3 three discrete "buffer regions" in thedata cache (although DC₋₋ ALG3b creates only 3 buffer regions,additional buffer regions could be added if desired).

FIG. 7 shows an illustrative computer program for implementing DC ALG3b.Referring to FIG. 7, DC ALG3b derives optimal values for sequentialreads (S_(b-sr)), random reads (S_(b-rr)), sequential writes (S_(b-sw)),and random writes (S_(b-rw)) in the same manner as DC₋₋ ALG3a. For theheterogeneous cache geometry, however, these four values define three(3) regions of the data cache with an optimal cache buffer size (S_(b) 0!, S_(b) 1!, S_(b) 2!) for each region.

First, DC ALG3b merges the sequential read size and the sequential writesize into one region because they share the same tendencies towardsbuffer cache size. The merged optimal value (S_(b-srew)) is set to thelarger of the optimal sequential read value (S_(b-sr)) and the optimalsequential write value (S_(b-sw)). The three regions defining the cachegeometry are then set as follows: region 1 (S_(b) 0!) is set to themerged optimal value (S_(b) 0!=S_(b-srsw)); region 2 (S_(b) 1!) is setto the optimal random read value (S_(b) 1!=S_(b-rr)); and region 3(S_(b) 2!) is set to the optimal random write value (S_(b) 2!=S_(b-rw)).

Once the buffer size for each of the three regions is set, the algorithmmust determine how much of the total data cache should be devoted toeach region. The algorithm first determines what percentage of theworkload is associated with each of the sequential reads (Percent SR),sequential writes (Percent SW), random reads (Percent RR), and randomwrites (PercentRW) in the same manner as DC₋₋ ALG3a. It then assigns tothe first region a percentage of the total data cache equal to the sumof the percentage of the workload associated with sequential reads andsequential writes (Percent 0!=PercentSR+PercentSW). The second region isassigned a percentage of the total data cache equal to the random readworkload percentage (Percent 1!=PercentRR) and the third region isassigned a percentage of the total data cache equal to the random writeworkload percentage (Percent 2!=PercentRW).

The Tuning engine then sends the server a list of the three cachebufferthree cache buffer sizes along with a list indicating what percentof total number of cache buffers each region should be assigned (send₋₋hetero₋₋ cache(S_(b) !, Percent !)).

4. Data Cache Flushing Intervals: DC ALG4 & DC ALG5

It is known in the art to "flush" a data cache periodically and/or uponthe occurrence of specified events in order to efficiently operate thecache. "Flushing" of the data cache involves moving through a list ofcache buffers and moving certain data to the host system's permanentstorage devices. In accordance with the present invention, flushes areactivated in order a) to satisfy a pre-defined (user-defined) criterionfor data integrity (i.e. periodic flushes), b) to maintain apredetermined ratio between "clean buffers" (which contain un-modifieddata) and "dirty buffers" (which contain modified data)("Clean/DirtyBuffer Ratio"), and c) to acquire usable data buffers needed for readsand writes ("Buffer Starvation").

The first type of cache flushing, the "periodic" flushing, is consideredto be a necessary part of any data cache mechanism. It counters thevolatility of the data in cache buffers with the stability andpermanence of storing the data in a permanent storage device. The tuningengine 40 leaves this flushing interval to the discretion of theserver's user. No tuning algorithms act on this flushing activity otherthan to simply differentiate periodic cache flushing rates from theoverall cache flushing rates. In this regard, the tuning engine 40monitors N_(p), which is defined as the moving average (over the fivemost recent sampling intervals (sampling interval=60 seconds)) of thenumber of data cache buffer flushes per sampling interval initiated by aperiodic flush mechanism in the server's data cache system.

a. Clean/Dirty Buffer Ratio: DC ALG4

The second type of cache flush, based upon the "Clean/Dirty BufferRatio," is controlled by DC₋₋ ALG4. This algorithm has a samplinginterval of 60 seconds and an action interval equal to ten samplingintervals (600 seconds). The following variables will be used indescribing the manner in which DC₋₋ ALG4 operates:

N_(sr) : the number of sequential reads performed by the server onbehalf of client requests during the last sampling interval.

N_(sr) !array holding sequential read data for all sampling intervals(zeroed out after each action interval).

N_(rr) : the number of random reads performed by the server on behalf ofclient requests during the last sampling interval.

N_(rr) !: array holding random read data for all sampling intervals(zeroed out after each action interval).

N_(r-avg) : the moving average (over the ten most recent samplingintervals of the number of reads performed by the server system onbehalf of client requests.

N_(sw) : the number of sequential writes performed by the server onbehalf of client requests during the last sampling interval.

N_(sw) !:array holding sequential write data for all sampling intervalszeroed out after each action interval).

N_(rw) : the number of random writes performed by the server on behalfof client requests during the last sampling interval.

N_(rw) !: array holding random write data for all sampling intervals(zeroed out after each action interval).

N_(w-avg) : the moving average (over the ten most recent samplingintervals) of the number of writes performed by the server on behalf ofclient requests.

N_(f) : the number of data cache buffer flushes during the samplinginterval which were initiated by an asynchronous flush mechanism in theserver data cache system which tries to maintain a minimum ratio of thenumber of clean to the number of dirty buffers.

N_(f-avg) : the moving average (over the ten most recent samplingintervals) of the number-of data cache buffer flushes per samplinginterval initiated by an asynchronous flush mechanism in the file serverdata cache system which tries to maintain a minimum ratio of the numberof clean to the number of dirty buffers.

R_(c) : the current value of the clean-to-dirty-buffer-ratio threshold.

R_(prop) : the proposed optimum value of the clean-to-dirty-buffer-ratiothreshold.

DC₋₋ ALG4 serves two purposes: 1) to determine when the server isexcessively flushing the data cache buffers (i.e., the clean/dirtybuffer ratio is being violated); and 2) to determine an optimal value(which will minimize flushing) for the clean/dirty buffer ratio basedupon the server's workload. DC₋₋ ALG4 utilizes the workload valuesdetermined by RTMP for DC₋₋ ALG3(a & b) to determine the optimum valuesfor a clean-to-dirty-buffer-ratio threshold.

A data cache buffer is considered "dirty" if the data it contains hasbeen modified as a result of a client request. A data cache buffer isconsidered "clean" if the data it contains has not been modified as aresult of a client request. The optimum ratio of clean to dirty bufferswill vary as the workload environment varies. A ratio of R_(c) =40%tells the server to maintain a ratio of 40 clean buffers to every 60dirty buffers in the data cache and to flush data cache buffers toincrease the number of clean buffers when the ratio drops below R_(c).

Read requests will not cause flushing since they do not affect the cleanto dirty buffer ratio. In contrast, write requests, which modify cachebuffers, may cause the clean to dirty buffer ratio to drop below R_(c),thereby initiating buffer flushing. Therefore, DC₋₋ ALG4 monitors theread and write requests over time and proposes values for R_(c) whichwill tend to minimize cache flushing.

FIG. 8 shows an illustrative computer program for DC ALG4. The algorithmtracks the moving average of read and write requests from clients viathe real time workload monitoring conducted by the tuning system 1 andcalculates the optimum clean/dirty buffer ratio based upon these values.DC₋₋ ALG4 maps the clean to dirty buffer ratio threshold (R_(c)) tomirror the ratio of read requests to total read/write requests in theworkload spectrum. Thus, as the percentage of reads in the workloadspectrum increases, so does the threshold R_(c).

Referring to FIG. 8, the algorithm checks the flushing rate, the numberof sequential reads, sequential writes, random reads, and random writesduring each sampling interval (GetFlushingRateStats(&FlushStats)) andmaintains a flushes v. time graph over a ten sample interval(UpdateGraph(numsample, FlushStats.N_(f))). Every ten sample periods (if(NumSamples==10)), the algorithm calculates the average number of reads(N_(r-avg) =GetAverage(10 samples, & N_(sr) !, & N_(rr) !)), the averagenumber of writes (N_(w-avg) =GetAverage(10 samples, & N_(sw) !, & N_(rw)!)), and the average number of flushes (N_(f-avg) =GetAverage (10samples, N_(f))) . After these values are updated, the algorithm checksfor two anomalies: 1) no flushing activity during last ten intervals; 2)saturation behavior in the flushing rate.

If there have been no flushes during the last ten sampling intervals(N_(f-avg) ==NULL ), then the clean to dirty buffer ratio is notaltered.

DC₋₋ ALG4 checks the flushes v. time graph to determine whether there issaturation behavior in the growth of the flushing rate (CheckFlushRate()). If there is saturation behavior, the flag ExpCheckOn is set and thethreshold R_(c) is decreased as follows: R_(c) is set to a valueR_(prop), which equals N_(r-avg) /(N_(r-avg) +N_(w-avg)) if and only ifR_(prop) is less than R_(c). If R_(prop) is greater than or equal toR_(c), R_(prop) is set instead to 80% of R_(c). The value R_(prop) isthen sent to the server as the new value for R_(c). Moreover, once thethreshold tendency flag ExpCheckOn is set, the flushing rate ismonitored and the value R_(c) decreased in this manner during everysampling interval until the flag is cleared.

If no anomalies were detected (there were flushes and the flagExpCheckOn is not set) then the algorithm will determine an optimalvalue for R_(c) based upon the read/write stream and return a value forR_(c) equal to N_(r-avg) /(N_(r-avg) +N_(w-avg)) to the server(SendToSeNer(Rprop)).

b. Buffer Starvation: DC ALG5

The purpose of this algorithm is to monitor data cache availability andto maintain as close to 100% availability as possible. The samplinginterval of this algorithm is 60 seconds and the action interval is fivesampling intervals (300 seconds). The following variables will be usedin describing DC₋₋ ALG5:

N_(s) : the number of data cache buffer flushes per sampling intervalinitiated by the buffer starvation flush mechanism in the server datacache system.

N_(rw) : the total number of read and write requests serviced by theserver system during the last sampling interval.

CA: the percentage of time the data cache was available on demand forreads and writes during the last sampling interval: 100 ((N_(s)/N_(rw))* 100).

CA !: an array of the cache availability values from the five mostrecent sampling intervals.

CA_(avg) :the moving average of the cache availability values from thefive most recent sampling intervals.

The "data cache availability" value (represented as a percent), CA,measures the ratio of the number of times the data cache had to beflushed in order to free up a data cache buffer to satisfy a read orwrite request from a client to the total number of reads and writes fromclients during a sampling interval. As previously stated, the purpose ofDC₋₋ ALG5 is to maintain data cache availability at as close to 100% aspossible.

Suppose that, during the last sampling interval, there were five cacheflushes due to buffer starvation (buffer starvation occurs when thecache must be flushed to free up a data buffer to satisfy a clientrequest) and during that interval there were a total of 100 reads andwrites. The cache availability value calculated for that interval wouldbe 100-((5/100)*100) or 95%. This indicates that, for 95% (or 95 in thiscase) of the reads and writes which occurred during that interval, thedata cache offered the needed cache buffers on demand, but for 5% (5 inthis case) of the reads or writes which occurred during the interval,the requests had to wait while the cache was flushed to free up theneeded buffer(s).

The goal is to maintain a cache availability value of 100%. This isaccomplished primarily by maintaining an appropriately large data cachesize. In general, the size of the data cache is controlled by DC₋₋ ALG2.DC ALG5, however, monitors the data cache availability, and upondetecting a degradation in availability, issues an alarm to DC₋₋ ALG2which causes DC₋₋ ALG2 to preempt its normal activity and take specifiedaction to alleviate the degradation in cache availability. So DC₋₋ ALG5tunes the server system's data cache by monitoring the cacheavailability, CA, and, when necessary, feeding an asynchronous alarminto the general data cache size tuning algorithm (DC₋₋ ALG2 :data cachesize routine()).

DC₋₋ ALG5 will be explained with reference to the illustrative computerprogram shown in FIG. 9. The algorithm keeps a window of the five mostrecent values of the cache availability (CA 1!-CA 5!) and determineswhether there is a cache availability problem by examining the averageof these values (CA_(avg) =GetAvgAvail(CA !)).

An average cache availability of over 90% is considered acceptable.Therefore, the flag IsStarving is set to false when CA>90 and no actionis taken.

An average cache availability of less than 80% indicates a degeneratecondition requiring immediate correction. Therefore, the algorithm setsthe flag IsStarving to TRUE, the flag StarveCase to DEGENERATE, andsends these values asynchronously to DC₋₋ ALG2. Referring to FIG. 4,DC₋₋ ALG2 checks for asynchronous messages from DC₋₋ ALG5 during everysampling interval (CheckForStarvation()). Since IsStarving is true andStarveCase is DEGENERATE, DC ALG2 immediately jumps to case (a) which,as discussed previously, implements a 100% increase in the cache size.

An average cache availability of between 80% and 90%, while stillconsidered unacceptable, does not require such a severe response.Referring to FIG. 9, a cache availability in this range causes DC₋₋ ALG5to set the IsStarving flag to TRUE and the StarveCase flag toNONDEGENERATE and to send these values asynchronously to DC₋₋ ALG2.Since the IsStarving flag is true, DC₋₋ ALG2 will interrupt its normalprocessing. However, since the StarveCase flag is not set to DEGENERATE,DC₋₋ ALG2 will not jump to case (a). Instead, it will correct the datacache size using its normal criterion (which is based on the hit ratio).However, as a result of the asynchronous signal from DC₋₋ ALG5, DC₋₋ALG2 will act immediately rather than waiting for its normal actioninterval.

II. Macro-level Algorithms

The macro-level algorithms monitor the overall affect of the dynamictuning implemented by the micro-level algorithms and provide the userwith information regarding the overall functioning of the server and theeffectiveness of the micro-level algorithms.

A. Performance Degradations/Saturations: MaLP-1

This macro-level algorithm assimilates all of the activity at themicro-level into a phase-space surface which is then used to:

a) judge the effectiveness of the micro-level activity;

b) present the micro-level activity to the user in a simple and coherentmanner; and

c) formulate propositions regarding future workload demands and serverperformance.

This phase-space surface is a three dimensional representation of theinteraction among the workload, the server system, and the micro-leveltuning algorithms (if they are in operation). The surface represents anx-y-z parametric phase space spanned by: an x-axis of valuesrepresenting the workload, a y-axis of values representing a systemresource characteristic such as CPU utilization, and a z-axisrepresenting a performance characteristic of the server such as asystem-generated value of the overall average response time of theserver to a given workload. This surface associates a particularworkload demand with a corresponding system resource expenditure andwith a resulting performance metric. Any point in time, past, present,or future (to a certain degree) can be located on this surface by thevector associated with the triplet (Workload, System Resource,Performance). All of the micro-level activity is necessarily assimilatedinto this point. In fact it is the hidden micro-level activity by thetuning engine's dynamic tuning which modifies the shape of this surfacethrough the server's response to workload demands.

Motion along this surface is analyzed by using standard mathematicaltechniques. This type of analysis allows the tuning system 1 to locateworkloads in the phase space and identify its associated system resourceand performance values. The surface can be generated by actual workloadsand/or artificial workloads.

The average response time of the server is measured by a probe which islocated in the tuning engine 40. As explained infra, the probe is asmall application level program which performs selected primitiveoperations on the server and measures the response time of the server tothose operations. Since the probe is an application level program, theresponse time will approximate the actual response time of the server toclient requests. While the server is operating under an actual workload,the probe performs primitive operations on the server and the responsetime of the server to those operations is used to generate an averageresponse time of the server for that particular workload. The resourceconsumption value for the workload is sampled from the server. In thismanner, a three dimensional phase space is generated using actualworkloads.

In order to form a more comprehensive phase space, however, the presentinvention provides additional probes in various clients in the networkand uses these probes, along with the tuning system 1 probe, to generatean artificial workload. During a period of low network activity, e.g.midnight, the present invention will use the probes to generate a widespectrum of workload values and then measure the resulting responsetimes and resource consumption. In this manner, the present inventioncan establish a fairly comprehensive surface around any particularworkload. The system can then use this surface to answer performance andcapacity questions as functions of workload for any server system forwhich a corresponding phase space has been generated.

1. The X-AXIS: Workload Characteristic

An enumeration technique implemented by the tuning system 1 provides aone-to-one mapping of the workload to a unique identifier value which isused to identify any particular workload on the graph. The tuningsystem's definition of a workload is based upon the average arrivalrates of file server primitives (which are sampled from the server viaRTMP). During a sampling interval, the tuning system 1 uses its RTMP(workload) to tabulate the average arrival rates of the followingprimitive requests from clients: sequential and random reads, sequentialand random writes, file opens, file closes, directory searches,byte-range locks, file creates, and "other". These values form a vectorwhich defines the workload for that interval: (n_(seq) read, n_(ran)read, n_(seq) write, n_(ran) write, n_(open), n_(close), n_(create),n_(lock), n_(search), n_(other)). Any workload, either real orartificial, can be described by a vector of this form. The tuning system1 takes a given workload vector and maps it to an unique value using amapping function.

The mapping function takes a workload vector in R^(n) (n equals thenumber of workload primitives associated with the workload vector) andmaps it to a scalar in R¹. The scalar is then taken as the "x"coordinate in the MaLP-1 phase space. The mapping function is needed tomap vectors which are "close" in R^(n) to scalars which are "close" inR¹. Collisions (i.e., two vectors in R_(n) mapped to one value in R₁)are allowed if the mapping function produces collisions only withworkload vectors which are very similar (i.e., the colliding workloadsrepresent similar workloads in terms of the distributions andfrequencies of the primitive operations represented by the components ofthe vectors).

The following mapping function used in the tuning system 1 produces thedesired results. The tuning system 1 utilizes the inner product of twovectors defined as <M,N>=||M|| ||N|| cos(theta), where "theta" is equalto the angle between the vectors M and N.

"Theta" is the angle between the vectors in R^(n) as well as R² becauseR^(n) is essentially "two dimensional" in the sense that the system isconcerned with only two vectors, M and N. These two vectors, althougheach has n coordinates, together span a plane subspace inside R^(n).This subspace can be regarded as basically the same as the standardplane R² ; the subspace is just a "copy" of R² tilted inside some largerR^(n). The tuning system 1 takes each workload vector in R^(n) andcomputes, using the inner product, the angle "theta" between theselected workload vector and a reference vector (which defaults as thebasis in R^(n) of (1,0,0, . . . . 0)). This value of "theta" is themapping of the workload vector from R^(n) to the R¹ phase space ofMaLP-1. The following procedure is repeated for each workload vector inR^(n) while maintaining the same reference vector in R^(n).

i) select a reference vector in R^(n) : the default is the R^(n) basis(1,0,0, . . . 0)=Y

ii) select a workload vector in R^(n) : Wa=(w₁,w₂, . . . , W_(n))

iii) compute the angle "theta" between W_(a) and Y by solving: ##EQU1##W_(a) =a vector which defines the workload at time t_(a) W_(a) =(SR_(a),RR_(a), SW_(a), RW_(a), O_(a), C_(a), S_(a), L_(a), Cr_(a), Ot_(a))

W_(a) =→Mapping Function→X_(a)

X_(a) =an unique value to which is mapped W_(a) in such a way that W_(a)is to be the only workload which is mapped to X_(a) : ##STR1## iv) if,after mapping all of the workload vectors in R^(n) onto the R¹ workloadaxis in the MaLP-1 phase space, the system detects collisions (differentworkload vectors mapped to the same value in R¹), the tuning system 1must verify that the collisions are of "similar" workloads (workloadswith similar distributions and frequencies of component primitiveoperations). If 95% or more of the collisions are of this type, thetuning system 1 will continue with the analyses since it is permissiblefor similar workloads to collide. Similar workloads should fall near toeach other in R¹. Collisions of "similar workloads" are treated asextreme cases where the distance between points approaches zero.

v) if more than 5% of the collisions are of "dissimilar" workloads(workloads whose distributions and frequencies of component primitiveoperations are quite different in pattern), the tuning system 1 willchange the reference vector used in the calculation of the "theta"values to a different basis in R^(n) (there are n of these referencevectors); recompute the mapping values of "theta;" and recheck forcollisions as in "iv" and "v". If, after exhausting the referencevectors of bases, the unfavorable collision condition persists, thetuning system 1 will calculate the center of mass in R^(n) of theworkload vectors (assigning each vector a unit mass) and use this centerof mass vector as the reference vector in calculating the "theta"values. If the unfavorable collision condition persists, the tuningsystem 1 will use the "theta" mapping with the least percentage ofunfavorable collisions.

2. The Y-AXIS: System Resource Characteristic

The tuning system 1 associates to every workload a correspondingconsumption of system resources needed to satisfy the workload demands.The system incorporates into its RTMP's a monitoring process whichtracks system resource consumption. Each workload vector in R_(n) (andR₁ as a result of the workload mapping function) has a singlecorresponding resource consumption vector which is plotted on theY-axis. Thus, the system maps to each workload a resource consumptionvalue which the system tracks as it tracks workload demands. Forexample, if CPU utilization is used as the resource characteristic, thepercent CPU utilization is plotted on the Y-axis as a function ofworkload demand.

3. The Z-AXIS: Performance Metric

The z-axis represents the macro-level manifestation of the micro-levelactivity of the tuning algorithms. In an illustrative embodiment of thepresent invention, the tuning system 1 uses the average response time ofthe server to client requests (RT), which is measured by the probe(s) asdiscussed below, as the performance metric mapped on the z-axis. In thiscontext, "hills" in the phase space represent performance degradations(higher average response times) in the server system and "valleys"represent performance improvements (lower average response times).

4. Performance/Capacity Analyses Using The Phase Space

In accordance with the present invention, one or more "probes" are usedto measure the response time of the server under differing workloads.The probes are small application programs that perform selectedprimitive operations on the server and measure the response time of theserver in response to commands from the tuning engine 40. In general,probes will be provided in the tuning engine 40 and in several clientsin the client population. It is possible, however, to use only a singleprobe located, for example, in the tuning engine 40. In the illustrativeembodiment of the present invention shown in FIG. 1, a central probe 95is located in the tuning engine 40 and client probes 98 are located invarious clients 10 in the client population 20. The tuning system 1 cansend a probe to a client by utilizing, for example, a PATHWORKS"Showserv" utility (a product of Digital Equipment Corporation).Alternatively, the probes can be manually installed in a client via theciient's disk drive.

The probes are generally application programs which run in a client thatis simultaneously performing its normal functions (e.g. running aWordperfect™ word processing program). However, a probe could also be adedicated hardware device capable of responding to the commands of thetuning system 1. In any case, since a probe is an application running inthe client (or a device which simulates an application running in aclient), the response time measured for a server is the actual responsetime that would be seen by an application in the client, i.e. the actualamount of time it would take to satisfy client requests.

The probes serve a dual purpose in the tuning system 1. While the systemis operating under an actual workload generated by client requests, theprobes monitor the response time of the server to that workload. Sincethe probes only implement a few primitive operations, the workloadgenerated by the probes does not significantly tax the resources of theserver. Therefore, the measured response time accurately reflects theresponse of the server to the client generated workload. In addition,the probes can be used to create an artificial workload by increasingthe frequency and magnitude of the probe's primitive operations. In thismanner, the probes can be used to map out an entire phase space for agiven server. The nature of the probes and the manner in which theygather response time information is discussed in detail in copendingapplication Ser. No. 08/050,929 which is hereby incorporated byreference.

The tuning engine 40 can assign a set of probes in the network to act asloading stations on a given server system. By issuing commands from thetuning system 1, the present invention can regulate the operation suites(sequences and average arrival rates of file server primitiveoperations) directed at the server in such a manner that the control.led stream of operations define a workload over a period of time. Inother words, the operation suite over a period of time can becharacterized by a Workload Vector (SR, RR, SW, RW, O, C, S, L, Cr, Ot)which is reduced to a unique identifier value W_(nid), a System ResourceVector (ex: CPU% consumed to satisfy the work demands over the timeinterval), and a Performance Vector (ex: average overall Response Time).Thus, for a given workload generated by the set of probes, a vector isgenerated using the resource and performance metrics previouslydescribed: Workload_(n) →(W_(nid), CPU_(n), RT_(n)).

This vector (W_(nid), CPU_(n), RT_(n)) is then plotted in the phasespace. The Workload Vector (SR, RR, SW, RW, O, C, S, L, Cr, Ot) is thenadjusted to represent a different workload demand: WorkloadVectornew=(SR+dSR, RR+dRR, SW+dSW, RW+dRW, O+dO, C+dC, S+dS, L+dL,Cr+dCr, Ot+dOt) and the process is repeated with the resultant vector(W_(newid), CPU_(new), RT_(new)) plotted in the phase space. In thisway, the tuning system 1 can, over a period of time, generate a set ofvectors in the phase space which can be used to determine athree-dimensional surface topology of the performance/capacitycharacteristics of the server. In a network which includes severalservers, the procedure can be repeated for each server on the network,resulting in characteristic surfaces for each server.

During the normal periods of operation for any given server, the tuningsystem 1 will update the phase space with new data vectors which arecomposed of workload, resource, and performance metrics gathered bymuting system 1 in real-time. These vectors are added to the phase spaceand the surfaces are updated to reflect the constant assimilation ofreal-time information. Moreover, the system can provide the user with aperformance trajectory of the server which shows the path that theserver (in terms of workload/capacity/performance) has taken across thesurface.

Because the surface topology is in part reflective of the application ofthe tuning engine's tuning recommendations to the server, the "base"topology of the surface might change significantly if the tuning changesmove the server to a state which is drastically different(performance-wise) than previous states. In this case, sections of theperformance trajectory through the phase space (the path of actualvalues across the surface) as well as pares of the general surfacetopology will be invalidated. This would necessitate a redefinition ofthe phase space surface by repeating the original sequence of operationsuites from the client probes.

However, it is entirely possible that only a subset of the surface issignificantly impacted and needs redefinition. To address this problem,the tuning system 1 periodically validates a set of randomly selectedworkloads in the phase space. If the projections of the vectors(Workload, Resource, Response Time) for these selected workloads are notsignificantly different than their projections during the previousvalidation period, the tuning system 1 concludes that theperformance/capacity phase space is still valid despite the dynamictuning of the server. If, however, the vector projections aresignificantly different than the projections during the previousvalidation period, the system notifies the user of the metric ofdifference and asks the user if the system should continue its analysesor, in the alternative, begin the surface redefinition process.

5. Interpreting The Phase Space Surfaces

The phase space surface is a macro-level representation of theunderlying relationships among workload demands, resource consumption,performance characteristics, and applied tuning. Any workload definedduring the operation of a server while it was being monitored by thetuning system 1 will be locatable in the phase space. In addition to thereal-time workloads, probe-driven workloads are plotted which addcomprehensiveness to the surface. The tuning system 1 locates thecurrent actual workload and tracks the server's motion over the phasespace surface as the actual workload changes with time (the performanceor workload trajectory).

By examining the contours of the phase space surface around the path ofthe workload trajectory (the path of actual workloads over time), thetuning system 1 can address questions regarding performance. Since thetuning recommendations from the tuning system 1 to the server help toshape the performance and resource consumption dimensions in the phasespace, the tuning system 1 can, by examining the immediate geometryaround the point on the surface representing the current workload, andthe direction the workload has been moving over time, attempt to predictthe server's performance with respect to a future workload. Once thesystem predicts a future workload, it locates this projected point inthe phase space, and uses this phase space position to estimate what theperformance characteristics will be if the actual workload reaches thisestimated position. In short, the tuning system 1 uses this methodologyto demonstrate to the user where the system has been, where the systemis, and where the system seems to be heading.

B. Examining Entropy in the File Server System: MaLP-2

In the context of the present invention, "entropy" denotes a measure ofthe ability of the server 30 to maintain a consistent level of serviceto the clients 10 despite changes in the workload. The micro-levelactivity within the server 30, which is generated in reaction to thechanges in the workload, is manifested at the macro-level as performancemetrics. These performance metrics can be used to quantify the entropywithin the server system. In accordance with an exemplary embodiment ofthe present invention, the amount of "entropy" in the server system isquantified by calculating a value proportional to the entropy of theserver system based upon the performance history over time(time-variant) of the service being provided to the clients 10. In thecontext of the present invention, a higher entropy value indicates ahigher degree of internal disorder in the server system with regard toits ability to process client requests in a consistent, timely, coherentmanner.

The MaLP-2 macro-level algorithm uses the average response time (theaverage, at a time, t, of the response times of all server primitiveoperations measured by the probes) to generate a two-dimensional, R²,phase space which is used to examine entropy patterns in serverbehavior. Techniques for constructing phase-space pictures from theobservation of a single coordinate of any dissipative dynamical systemare discussed in Packard, et al., "Geometry from a Time Series",Physical Review Leners, 45/9, 1980.

MaLP-2 monitors the average response time of the server over time andgenerates a two dimensional graph representative of the entropy of theserver. Referring to FIG. 10, MaLP-2 monitors entropy in the server byexamining pairs of response times values; i.e. two consecutive responsetimes as measured by a single probe. The algorithm generates a graphwhose X-axis is the most recently sampled average response time, RT₁,and whose Y-axis is the preceding response time, RT₂. Therefore, if thetuning system 1 gathers n average response time values, the points willbe plotted on the graph as follows:

    ______________________________________                                        (RT 1!, RT 1!)                                                                (RT 2!, RT 1!)                                                                (RT 3!, RT 2!)                                                                .                                                                             .                                                                             (RT n!, RT n-1!)                                                              ______________________________________                                    

Referring to FIG. 10, the system phase space of the graph is dividedinto grid boxes 200 as follows:

First, the system must establish a response time saturation value. Theresponse time saturation value RT_(sat) is defined as the response timeat which the server begins to exhibit saturation behavior. RT_(sat) canbe determined for any given server by utilizing the probes toartificially alter the server's workload and measuring the resultingresponse time to determine the point at which the server's performancebegins to exhibit saturation behavior. The manner in which saturationbehavior is identified in discussed below in Section II(C).Alternatively, RT_(sat) can be defined as any response time which thesystem manager or central user determines to be a maximum permissiblevalue.

The system then defines a unit of length, RT_(len), where RT_(len)=RT_(sat) /n. While "n" can be set to any value, it will be defined asn=10 for purposes of this discussion. Thus, RT_(len) =RT_(sat) /10. Thegraph is divided into grid boxes having sides of length RT_(len). Eachgrid box defines a "macrostate."

As response time values are received by the tuning system 1, responsetime pairs (RT1, RT2) are plotted on the graph. Since the server'sresponse time should be fairly consistent over time, all the points onthe graph, under optimal conditions, should fall into a singlemacrostate (grid box). Therefore, the entropy of the system increases asthe number of occupied macrostates increases.

The tuning system 1 assigns each grid-box an id, "i", starting from O to"n" thereby creating macrostates "0" through "n." From the plot of theordered pairs (RT_(t), RT_(t-1)) overlaid onto the grid box, the numberof points in each grid box is counted and the ratio of (number of pointsin grid box "i")/(total number of points) is taken as the probability,P_(i), of finding the server in macrostate "i."

The entropy, S, is proportional to:-(SUM_(i) (P_(i) log P_(i))). Sincethe present invention is concerned with detecting an increase in entropyand not in determining an actual value for the entropy, the constant ofproportionality is ignored for purposes of this analysis and S=-(SUM_(i)(P_(i) log P_(i))).

The system tracks the entropy values over time and stores these valuesas a set of ordered pairs (t, S_(t)). The ordered pairs are plotted on agraph of S v. t and monitored for indications of saturation behavior. Ifsaturation behavior is detected, the system user is notified of apotential problem in the server. The user can then examine the storedresponse time and entropy values and investigate the source of theproblem.

C. Capacity Degradations/Saturations: MaLP-3

Another feature of the present invention sets an alarm for the centraluser or system manager when the response time of a server, considered asa whole, indicates that the server is approaching saturation behavior,i.e. a suddenly decreasing level of performance. This feature will beexplained with reference to FIGS. 11 and 12. The manner in which aserver's performance varies with its workload is shown in FIG. 11. Withregard to any server, there exists a point x,y after which the responsetime of the server begins to increase suddenly as the workloadincreases. This point is sometimes referred to as the "knee" of thecurve and indicates the beginning of saturation behavior. For obviousreasons, this is a very undesirable condition. Saturation behavior by aserver can be counteracted by altering the workload or server parametersto move the server's performance back into an area of linear behavior.If the tuning engine 40 is currently implementing dynamic tuning on theserver in question, saturation behavior indicates to the user that thetuning algorithms are not having the desired effect.

An exemplary method of determining the "knee" or saturation point of thecurve of FIG. 11 will now be illustrated. The workload of the server isvaried and the corresponding response times are calculated to form nworkload/response time pairs ((W₁, RT₁) through (W_(n), RT_(n))) whichcan be plotted on the graph of FIG. 11. The knee of the curve (pointx,y) is calculated as follows:

1) calculate an average slope:

    m.sub.avg =(W.sub.n -W.sub.1)/(RT.sub.n -RT.sub.1);

and

2) calculate n-2 local slopes, m₂ -m_(n-1), where

    m.sub.2= (W.sub.3 -W.sub.1)/(RT.sub.3 -RT.sub.1)

and

    m.sub.n-1 =(W.sub.n -W.sub.n-2)/(RT.sub.n- RT.sub.n-2)

The knee (or saturation point) of the curve is the one of the n points,x, which satisfies each of the following conditions m_(x) =m_(avg) ±5%;m_(x-1) <=m_(avg) ; and m_(x+1) >m_(avg).

In accordance with the present invention, a single probe is chosen(generally the probe in the tuning engine 40) and probe data with regardto the server is obtained as described previously. The tuning system 1determines when a server is approaching saturation behavior by examining"triplets" of measured response times (RT₁, RT₂, RT₃), i.e. threeconsecutive response times, from the probe. A response time saturationvalue RT_(sat) is defined as the response time at which the serverbegins to exhibit saturation behavior. As described above, RT_(sat) canbe determined for any given server by utilizing the probes toartificially alter the server's workload and measuring the resultingresponse time to determine the point at which the server's performancebegins to exhibit saturation behavior.

Two types of saturation response time values can be identified: a) asaturation point of the average response times where the aggregate valueis formed from a weighted average of the primitives with each primitivebeing assigned an equal weight (an absolute saturation value), or b) asaturation point of the average response times where the aggregate valueis formed from a weighted average of the primitives with eachprimitive's weight proportional to the primitive's relative frequency inthe server's workload distribution of primitive operations (a relativevalue). Which type of saturation value is used in the analyses isdecided upon by the system user. In any event, it is this weightedaverage of the response times of primitive operations which is used todetermine the RT_(sat) value. Alternatively, RT_(sat) can be defined asany response time which the system manager or central user determines tobe a maximum permissible value.

Referring to FIG. 12, there is shown a graph with RT₁, RT₂, and RT₃ asthe x, y and z axes, respectively. Each sequential set of 3 responsetime values creates a triplet as shown in FIG. 13. Each triplet forms asingle point on the graph. The maximum permissible response time forms acube with the length of the sides being equal to RT_(sat), as shown. Ithas been empirically determined that the set of triplets measured overtime will typically be bounded by a sphere of radius r_(b). The centerof the sphere (which also defines the center of the cube) can bedetermined, for example, by computing the arithmetic mean of the tripletvalues calculated over a period of time. The radius, r_(b), can then bedefined as the distance from the most recent triplet value (or from theaverage position of a set of recent triplet values) to the center of thesphere.

In accordance with an embodiment of the present invention, the sphere isformed as follows. First, the tuning system 1 must reduce the set ofpoints on the graph to a central point or "center of mass" in order togeometrically center the sphere and the bounding cube. For a set of "i"points distributed on the graph, each point (x_(i),y_(i),z_(i)) isdefined as having a unit mass (m_(i) =1). The center of mass(x_(c),y_(c),z_(c)) of these points is determined as follows:

i) x_(c) = (m₁ *x₁)+(m₂ *x₂)+. . . +(m_(i) *x_(i))!/(m₁ +m₂ +. . .+m_(i))

ii) Y_(c) = (m₁ *y₁)+(m₂ *y₂)+. . . +(m_(i) *y_(i))!/(m₁ +m₂ +. . .+m_(i))

iii) z_(c) = (m₁ *z₁)+(m₂ *x₂)+. . . +(m_(i) *z_(i))!/(m₁ +m₂ +. . .+m_(i))

The center of the sphere is then set to the center of mass(x_(c),y_(c),z_(c)).

The radius of the sphere is calculated as follows: The tuning system 1calculates the distance from (x_(c),y_(c),z_(c)) to the most recent datapoint in the distribution (x_(i),y_(i),z_(i)) using the distance formulain R³ : d_(i) =sqrt (x_(i) -x_(c))² +(y_(i) -y_(c))² +(z_(i) -z_(c))² !.This produces the distance from the center of mass of the distributionto the most recent position of the server in the phase space. The radiusof the sphere, r_(b), is set equal to this distance, r_(b) =d_(i), andthe equation of the sphere is defined as: (x-x_(c))² +(y-y_(c))²+(z-z_(c))² =r_(b) ².

MaLP-3 uses the volumes of the sphere and cube generated in this phasespace to determine the current and maximum capacity utilizations of theserver. The volume of the bounding cube is interpreted as an indicatorof the maximum capacity of the server to do work: (C_(max) =V_(cube)=(RT_(sat))³. The volume of the sphere is interpreted as an indicator ofhow much of the maximum capacity is being used at time "t": C_(t)=V_(sphere) =(4/3)*(pi)*(r_(b))³. Thus, the ratio of C_(t) /C_(max) isreported by MaLP-3 to the user as a metric of current server capacityutilization.

In accordance with the present invention, the rate at which the sphereis approaching the cube is identified and reported as the rate at whichthe server is approaching saturation behavior. MaLP-3 attempts todetermine a) the rate at which C_(t) is changing and b) the rate atwhich C_(t) is approaching C_(max). The bounding cube is set beco-centric with the capacity sphere at (x_(c),y_(c),z_(c)). Thus, boththe cube and the sphere are translated to a frame of reference about(x_(c),y_(c),z_(c)). Since (x_(c),y_(c),z_(c)) is constantly beingrecalculated by MaLP-3, this translation is performed at each samplinginterval.

The rate at which C_(t) is changing, dC_(t) /d_(t), is a measure of therate at which the server is using up its available resources. This rateis calculated as follows:

i) keep an array of the ordered pairs (C_(t), t) for a designated timeperiod of interest;

ii) find a "best-fit" curve through the graph of the ordered pairs suchthat f(t)=C_(t) ; and

iii) differentiate f(t) at the time in question to get the value ofdC_(t) /dt, i.e. the rate at which the volume of the sphere is changingat time t.

It is known that the rate at which the volume of a sphere is changingand the rate at which the radius of the sphere is changing are related.Thus it can be shown that:

    dC.sub.t /dt=(dC.sub.t /dr.sub.b)*(dr.sub.b /dt)

    dC.sub.t /dt=(4/3)*pi* 3*(r.sub.b)2*(dr.sub.b /dt)

so,

    dr.sub.b /dt=1/(4*pi*r.sup.b.sup.2)* dC.sub.t /dt

dC_(t) /dt, which was calculated in above, can be plugged into thisequation and the value for dr_(b) /dt (the rate at which the capacitysphere is changing with time) can be determined. The tuning system 1uses this value to predict a time, t_(sat), when the server will reachits predefined saturation point if the workload remains relativelyconstant. The tuning system 1 estimates that the saturation point willbe reached at the time, t_(sat), when the value of r_(b) changing at therate of dr_(b) /dt will equal the value of RT_(sat) (in other words whenthe sphere reaches tangency with the cube). Assume, for example, thatthe current value of r_(b) equals 5 ms, r_(sat) equals 6 ms and thecurrent value of dr_(b) /dt equal 0.1 ms/hr. Assuming the workloadremains constant, the server will reach saturation in 10 hours.Therefore t_(sat) =10 hrs.

As a result, the tuning system 1 can set an alarm indicating that theserver is approaching saturation behavior when t_(sat) drops below acertain threshold. The system will also set an alarm when the sphereintersects the cube to indicate when the server has entered thesaturation region. In a system including several servers, the abovedescribed process can be performed with respect to each server in thesystem thereby forming a separate graph and triggering a separate alarmfor each server. Moreover, since this process uses only the responsetime values generated by the probes, the process can also be performedon servers which are not configured to allow dynamic tuning.

What is claimed is:
 1. A method of operating an open file cache thatmaintains files open after their closing has been requested, each filebeing so maintained for an open file close delay W_(d) after its closinghas been requested, the method comprising the steps of:periodicallydetermining (1) a number F_(o) of open files, (2) a number F_(i) of idlefiles, (3) a number F_(a) of active files, and (4) a number F_(ia) offiles that have changed from idle to active during the most recentperiod; and for the following cases of the set of numbers {F_(o), F_(i),F_(a), F_(ia) }, performing the corresponding steps as follows: {0, 0,0, 0}: setting the value of W_(d) to be a default value W_(d0) ; {>0,0, >0, 0}: setting the value of W_(d) to be W_(d) 0 ; {>0. >0, >0, 0}:if the set of numbers {F_(o), F_(i), F_(a), F_(ia) } has equaled{>0, >0, >0, 0} for at least a first number of preceding periods, thenincreasing the value of W_(d) to be a random number greater than thecurrent value of W_(d) and less than (5*W_(d0)), and if the value ofW_(d) has been so increased for at least a second number of precedingperiods, then setting the value of W_(d) to W_(d0) ;{>0, >0, >0, >0}: ifthe value of W_(d) was not increased in the most recent interval, thenincreasing the value of W_(d) to be a random number greater than thecurrent value of W_(d) and less than (5*W_(d0)); and {else}: leavingW_(d) at its current value.
 2. A method according to claim 1, whereinthe first number of preceding periods is three and the second number ofpreceding periods is five.